In using power tongs to make and break joints between sections of a pipe string used in the oil industry, it is customary to provide a tong assembly that includes, generally, an upper powered tong for rotating an upper pipe section of the string, and a lower backup tong which clamps a lower pipe section to resist rotation. Both tongs have jaws which move radially to clamp a pipe section held within a throat portion of the tong, and the powered tong has means, usually hydraulically operated, for rotating its jaws. These two tongs are joined together by suspension means which resist rotation of the lower backup tong relative to the upper tong so that torque can be applied to the pipe joint held between the tongs, and the suspension means allows limited axial movement of the two tongs with respect to each other to accommodate axial movement between the pipe section as the joint is screwed together.
It is desirable that the tong assembly apply a predetermined torque to the joint, to prevent the joint being made too loose or too tight, and for this purpose the torque being applied to the joint may be measured throughout the process of making or breaking joints. This may be accomplished by introducing a torque-measuring device into the suspension means that extends between the upper and lower tongs. In order for this torque-measuring device to operate accurately, substantially all of the torque developed between the two tongs should pass through the measuring device.
It is customary to support the tong assembly by suspending the upper, powered tong from chains or cables connected to a crane hook. Additionally, in the past, short chains have been used as part of the suspension means for the lower tong, these short chains extending between the throat regions of the upper and lower tong. The rear portion of the backup tong, remote from the throat, has in the past been supported by a stiff, but twistable, rear leg or post providing a link between the rear portions of the tongs through which torque developed by the assembly can be measured.
With the use of chains as part of the suspension means between the upper and lower tongs, all of the torque developed by the assembly passes through the twistable rear leg or post (hereinafter the “post”) and is measurable by the torque-measuring device. However, a drawback of the use of chain-supports of this type is that when the tong assembly is moved aside from the wellhead for storage, as by being deposited on the ground or other support surface, then, in the absence of a lifting force provided by the crane hook, the upper, powered tong is only supported by the post at the back. This leaves the upper tong supported in a cantilevered manner that may require special reinforcement of the post and post/tong connections or otherwise expose it to damage.
In more recent designs the suspension means for connecting the upper and lower tongs has included legs or struts extending upwardly from the backup tong. These struts replace the short inter-tong chains referenced above. Instead the struts extend from adjacent the backup tong throat, to support the powered tong adjacent its throat. Usually, these struts have been rigidly connected to the upper tong, while lower portions of these struts have passed through apertures in the backup tong and have terminated below the latter tong, the struts supporting the backup tong by means of compression springs extending between the backup tong and the lowest ends of the struts. These springs allow a degree of lateral motion to occur between the struts and the lower tong, and therefore between the upper and lower tongs. As well they allow axial motion to accommodate the relative axial movement between pipe sections as they are joined. Such an arrangement is shown for example in U.S. Pat. No. 6,334,376, which issued Jan. 1, 2002 to Torres. Because the struts extend to below the bottom of the lower tong, when the assembly is set-down on the ground or a deck, the struts provide adequate support for the upper tong in such conditions.
The two tongs will, when applying torque to pipe sections, endeavour to displace themselves laterally with respect to each other in reaction to the torque being applied. This lateral displacement tendency is resisted at the twistable post that carries the torque sensing means. While it is desirable to minimize the amount of this lateral displacement, some displacement must necessarily occur in order for torque to be conveniently measured.
With the suspension means using the struts, e.g. as shown in the aforesaid Torres patent, under high torque conditions a strut passing through an aperture in the backup tong may contact the side of such aperture. When this occurs, a portion of the torque being developed between the upper and lower tongs is absorbed in the strut-to-backup tong contact. This means that the torque measuring device is not measuring the true torque developed between these two units.
A system is required that will provide for the support of the components within a power tong assembly, while minimizing the development of torque transmitted through other means than the torque measuring device.